1. Field of the Invention
The Invention relates to wind turbines for production of electrical power. The invention is particularly applicable to wind generators that may be used in locations where high average winds are not expected but that nonetheless must survive adverse weather conditions and loadings specified by applicable building codes for wind turbines.
2. Description of the Related Art
The potential for wind power energy generation for a location is measured by a scale referred to as the ‘wind power class.’ There are seven ‘wind power classes,’ ranging from class 1, with annual average wind speeds of less than 5.6 meters/sec at an elevation of 100 meters, to class 7, with annual average wind speeds of 8.8 meters/sec and above at an elevation of 100 meters. Much of the United States (and the world) has relatively low average wind speeds. For example, much of the Commonwealth of Pennsylvania falls within classes 3 or 4, with annual average wind speeds between 6.4 to 7.5 meters/sec at an elevation of 100 meters.
The wind power industry has focused development of wind power in areas of relatively high average wind speed. The prior art wind generators currently available were developed for such areas. Such generators generate only a fraction of their nameplate rating when used in areas of lower average wind speed.
Current-technology wind generators generally are horizontal up-wind wind turbines, with the axis of rotation of the blade horizontal to the ground and the rotating blade upwind of the supporting tower. The forces acting on the blade must be reacted (that is, opposed and supported) by the blade, blade root, hub including bearings, drive system and tower. Those forces include the steady force resulting from the weight of the blade and the varying force of the wind. The force of the wind acting on the blade varies along the span of the blade and varies with each revolution of the blade. Gusts, variable wind speeds and inclement weather can place a very high loading on the structures of a wind turbine. Wind turbines also are subject to frequent starting and stopping and loads.
The traditional approach to the problem of designing a wind turbine that will survive inclement weather is to select a design that is very rigid and strong. In current technology wind turbines, the blade, hub and tower are rigid. The problem with this approach is that the resulting turbines are inefficient at low wind speeds, bulky, heavy, expensive, difficult to build and difficult to transport. The blades, blade roots, bearings, hubs and power transmission system are areas of particular concern.
For large current-technology wind turbines, the gear sets and bearings are very large, with some gears being several feet in diameter. Such large gears can be produced by very few suppliers using very expensive, custom-built equipment. Large current-technology wind turbines cannot be easily transported by road due to the size and bulk of the components, particularly the tower and blades. Erection of such a turbine is a significant construction project requiring use of heavy equipment and large cranes.
A horizontal up-wind wind turbine must include a yaw bearing connecting the nacelle to the tower to support the nacelle and turbine blades and to turn the axis of rotation, and hence the blades, into the wind. Current technology wind turbines utilize a single large-format yaw bearing located at the top of the tower. Because the turbine blades are very heavy and are supported by the nacelle at a distance from the yaw bearing, the local forces acting on the single yaw bearing are multiplied by the lever arm created by the nacelle. Current technology yaw bearing must be very large, strong, and expensive to support the resulting bending moments.
Because the turbine blade rotates, the relative speed and hence relative direction between the turbine blade and the wind varies along the length of the blade from the root to the tip. The blade therefore must have a twist to achieve the optimum angle of attack of the rotating blade. Because of the size of the blades, the blades must be larger at the root end to provide adequate strength to support the resulting weight.
The low-wind speed turbine designer is faced with conflicting design goals—design a turbine that efficiently extracts power from the wind in low wind conditions but that can also survive the loads, both steady and cyclic, imposed by inclement weather.
The prior art does not teach the wind turbine of the Invention.